<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style type="text/css">
body>div {
	width: 200px;
	height: 200px;
	background-color: red;
	margin: 100px 0 0 100px;
	overflow: hidden;
    /* 如果想让绝对定位的元素坐标偏移相对于
    某一个上级元素,需要在上级元素中添加相对定位  */
    position:relative;
}

body>div>div {
	width: 100px;
	height: 100px;
	background-color: green;
	margin: 50px 0 0 50px;
	overflow: hidden;
}
div>div>div{
  width: 50px;
  height: 50px;
  background-color: blue;
  margin: 20px 0 0 20px;
  position: absolute;
  top: 0;
  left: 0;
}
</style>
</head>
<body>
  <div>
    <div>
      <div></div>
    </div>
  </div>
</body>
</html>